1. Field of the Invention
This invention relates to a photoelectric conversion device that converts a quantity of light received in each of a plurality of light wavelength ranges into an electrical signal, a method for manufacturing the photoelectric conversion device, and an image sensor provided with the photoelectric conversion device.
2. Description of Related Art
An image sensor is provided with a photoelectric conversion device that converts a quantity of received light into an electrical signal. For example, a color image sensor that reads a color image is provided with a photoelectric conversion device having sensors (photodiodes) for red, green, and blue, respectively. A color filter that transmits only light of a color to be detected is disposed on a light incidence surface of the sensor for each color, and a signal corresponding to the quantity of light incident through the color filter is output from each sensor.
However, since this photoelectric conversion device must form the filter in a process for producing the device, a large number of steps are required, and manufacturing costs rise. Therefore, a photoelectric conversion device that does not use a filter for each color has been proposed (see Japanese Unexamined Patent Publication No. 8-316521).
FIG. 8 is a sectional view illustrating a photoelectric conversion device that has no filter.
This photoelectric conversion device 100 includes a p-type substrate 101 made of, for example, silicon and a p-type epitaxial layer 104 formed thereon. A field oxide film 107 is formed on the epitaxial layer 104. The field oxide film 107 has thicker parts formed at predetermined intervals than the other parts. An n-type diffusion layer 105 and an n-type buried layer 102 are formed between the thicker part of the field oxide film 107 and the p-type substrate 101.
Accordingly, the epitaxial layer 104 is divided into a plurality of sections, which serve as sensor I, sensor II, and sensor III, respectively. In sensors I, II, and III, a p-type base region 106 is formed in the center of the surface part of the epitaxial layer 104.
A p-type buried layer 103 is formed between the p-type substrate 101 and the epitaxial layer 104 of sensors II and III. Accordingly, the thickness of the epitaxial layer 104 of sensor II and the thickness of the epitaxial layer 104 of sensor III are smaller than that of sensor I. The thickness of the epitaxial layer 104 of sensor II is almost equal to that of sensor III.
A light absorbing member 108 that is made of, for example, polysilicon and that absorbs blue light to some degree is formed on the field oxide film 107 on sensors I and II.
In sensors I, II, and III, when light impinges thereon, a quantity of carriers (electron-hole pairs) corresponding to the quantity of incident light are generated in the epitaxial layer 104, and photocurrent (photoelectromotive force) corresponding to the number of holes is taken out through the base region 106.
Herein, since the optical-absorption coefficient of the epitaxial layer 104 becomes smaller as the wavelength of incident light becomes longer, light that enters the epitaxial layer 104 from the surface thereof can reach a deeper place as the wavelength becomes longer. Therefore, if the epitaxial layer 104 is small in thickness, long-wavelength light (for example, red light) will not be sufficiently absorbed.
In the photoelectric conversion device 100, the epitaxial layer 104 of sensor I is formed to have a thickness capable of absorbing light in a wide wavelength range from red light to blue light, whereas the epitaxial layer 104 of sensors II and III is formed to have a thickness capable of absorbing light in a wavelength range chiefly from green light to blue light.
Attention will now be paid to the presence or absence of the light absorbing member 108 that absorbs blue light. Since the light absorbing members 108 are provided on sensors I and II, respectively, red light and green light enter the epitaxial layer 104 of sensors I and II. Therefore, sensor I generates a photocurrent corresponding to the quantity chiefly of red light and green light, and sensor II generates a photocurrent corresponding to the quantity chiefly of green light. On the other hand, since the light absorbing member 108 is not provided on the sensor III, red light, green light, and blue light enter sensor III. Therefore, sensor III generates a photocurrent corresponding to the quantity chiefly of green light and blue light.
Thus, since sensors I, II, and III differ from each other in a combination of red light, green light, and blue light, which are to be absorbed so as to generate a photocurrent, the quantity of red light, the quantity of green light, and the quantity of blue light can be calculated by arithmetic processing based on the magnitude of the photocurrent generated by sensors I, II, and III.
However, since the light absorbing member 108 is required to be provided even in the thus structured photoelectric conversion device, manufacturing costs could not be sufficiently reduced.
Additionally, it is necessary to connect signal extracting electrodes to the base regions 106, respectively. Therefore, openings to thread the signal extracting electrodes must be formed in the light absorbing member 108, thus causing an increase in cost.
Additionally, normally, a semiconductor layer to absorb light and generate carriers is depleted when the photoelectric conversion device is driven. Voltage needed for depletion becomes larger correspondingly with an increase in thickness of the semiconductor layer. Therefore, a large voltage is needed to deplete a semiconductor region thickened to absorb long-wavelength light, and hence a driving voltage of the photoelectric conversion device was large, and a driving voltage of an image sensor including the photoelectric conversion device was large.